Previous work from this section has demonstrated that the expression of murine mammary tumor virus endogenous sequences in cell culture is dramatically stimulated by glucocorticoid hormones, and that this stimulation is the result of an increase in the rate of viral RNA synthesis. Using high multiplicities of infection, we have now established a series of MMTV chronically infected cell lines derived from variants of the rat hepatoma cell line. Analysis of viral specific RNA levels in these strains indicates that the expression of this exogenously acquired viral information is also under the control of glucocorticoid hormones. To permit a detailed molecular analysis of the correlation between the structural organization of integrated viral information and the control of its expression, a major effort is underway to construct clones of integrated viral sequences utilizing recombinant DNA technology. The achievement of this goal requires the establishment of the requisite bacterial cloning technology and procedures for the enrichment of viral DNA. These goals have been achieved and the selection of viral specific recombinant clones from integrated DNA sequences is now underway.